Electrophilic reducing

Iodine azide, on the other hand, forms pure adducts with A -, A - and A -steroids by a mechanism analogous to that proposed for iodine isocyanate additions. Reduction of such adducts can lead to aziridines. However, most reducing agents effect elimination of the elements of iodine azide from the /mwj -diaxial adducts of the A - and A -olefins rather than reduction of the azide function to the iodo amine. Thus, this sequence appears to be of little value for the synthesis of A-, B- or C-ring aziridines. It is worthy to note that based on experience with nonsteroidal systems the application of electrophilic reducing agents such as diborane or lithium aluminum hydride-aluminum chloride may yet prove effective for the desired reduction. Lithium aluminum hydride accomplishes aziridine formation from the A -adducts, Le., 16 -azido-17a-iodoandrostanes (97) in a one-step reaction. The scope of this addition has been considerably enhanced by the recent... 

The most characteristic reaction of amine boranes is their conversion into aminoboranes and, subsequently, to borazine at higher temperatures.9,17 For complexes of low stability, the transfer of H from the boron to the donor is a characteristic process,11 40 as in the utilization of diborane as an electrophilic reducing agent. The neutral complexes of boranes are fairly stable towards hydrolysis. The key step of the hydrolysis was formerly assumed to involve displacement of BH3 by a proton, whereas in recent studies ionic intermediates, containing five-coordinated boron (R3N—BH4) are also taken into consideration.41,42 The hydrolytic... 

A detailed study of a series of a,a-halo ketones demonstrated that electrophilic reducing agents, notably borane, DIBAL-H and BH3 DMS, provided excellent yields of the alcohols without reduction of the halogen bond. In contrast, nucleophilic reagents (NaBH4, Li(Bu 0)3AlH and K-selectride) were unsatisfactory. The results for a representative a,a-dibromo ketone (87) being reduced to the mono- (89) and di-bromohydrin (88 equation 22) are shown in Table 4. 

The a-oxoketone dithioacetal reduction is reported [9] to follow different pathways, depending on the nature of reducing agents and reaction condition (Chart 25.8). The electrophilic reducing agent, 9-BBN effects the conjugate reduction to afford the p-methylthioketone [10]. 

Another variation of the Madelung synthesis involves use of an O-alkyl or O-silyl imidate as the C2 electrophile. The mechanistic advantage of this modification stems from avoiding competing N-deprotonation, which presumably reduces the electrophilicity of the amide group under the classical conditions. Examples of this approach to date appear to have been limited to reactants with a EW substituent at the o-alkyl group[15,16]. 

The 2-nitrothiazole can be reduced to the corresponding aminothiazole by catalytic or chemical reduction (82, 85, 89). The 5-nitrothiazole can also be reduced with low yield to impure 5-aminothiazole (1, 85). All electrophilic substitution reactions are largely inhibited by the presence of the nitro substituent. Nevertheless, the nitration of 2-nitrothiazoIe to 2,4-dinitrothiazole can be accomplished (see Section IV). 

Reduction of arenes by catalytic hydrogenation was described m Section 114 A dif ferent method using Group I metals as reducing agents which gives 1 4 cyclohexadiene derivatives will be presented m Section 1111 Electrophilic aromatic substitution is the most important reaction type exhibited by benzene and its derivatives and constitutes the entire subject matter of Chapter 12... 

Arylamines contain two functional groups the amine group and the aromatic ring they are difunctional compounds The reactivity of the amine group is affected by its aryl substituent and the reactivity of the ring is affected by its amine substituent The same electron delocalization that reduces the basicity and the nucleophilicity of an arylamme nitrogen increases the electron density in the aromatic ring and makes arylamines extremely reactive toward electrophilic aromatic substitution... 

Acylation. Acylation is the most rehable means of introducing a 3-substituent on the indole ring. Because 3-acyl substituents can be easily reduced to 3-aLkyl groups, a two-step acylation—reduction sequence is often an attractive alternative to direct 3-aLkylation. Several kinds of conditions have been employed for acylation. Very reactive acyl haUdes, such as oxalyl chloride, can effect substitution directiy without any catalyst. Normal acid chlorides are usually allowed to react with the magnesium (15) or 2inc (16) salts. The Vilsmeier-Haack conditions involving an amide and phosphoms oxychloride, in which a chloroiminium ion is the active electrophile, frequentiy give excellent yields of 3-acylindoles. 

Heterocychc A/-oxides can react at the oxygen atom with a variety of electrophilic reagents to give adducts which, according to the reagent and reaction conditions, may be stable or react further (39). Heterocychc A/-oxides are reduced by reaction of nucleophiles at the N-oxide oxygen. 

---